net: phy: Fix typo for MDIO module boilerplate comment

The module boilerplate macro is named mdio_module_driver and not
module_mdio_driver, fix that.

Fixes: a9049e0c ("mdio: Add support for mdio drivers.")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>